An internal-combustion engine like the one in your Subaru burns a mixture
of gasoline and air. The gasoline is delivered to the engine from the fuel tank,
and the air it mixes with is delivered by the intake manifold. As your Subaru tackles
everything from high-speed highway passing lanes to stop-and-go traffic, its engine
breathes the air it needs through its intake manifold.
Delivering air to the cylinder efficiently is the job of the intake manifold, says Kazuei Itoh of the Subaru Engineering Division. A lot of the engines performance is determined by it.
HOW IT WORKS
In the era before fuel injection, gasoline was mixed with air in the carburetor, and the resulting air/fuel mixture was delivered to the engines cylinders by the intake manifold.
Now that more efficient computer-controlled fuel-injection systems have long since replaced the carburetor, the intake manifold delivers only air. Today the fuel injection system delivers gasoline much closer to the cylinder at the base of the intake manifold next to the intake valves of the cylinder head where it mixes with air from the intake manifold to become an extremely precise mixture of air and fuel.
The intake manifold of the naturally aspirated (non-turbocharged) Subaru horizontally opposed engine is positioned on the upper part of the engine. The air flows from the air intake (located in the front of the engine compartment) through the air cleaner and into the intake manifold. This is where the throttle valve adjusts and measures the total volume of air. When maximum power is required, the valve is fully open. At idle, the valve is closed, and a tiny amount of air ported around the throttle valve keeps the vehicle from stalling.
Once drawn into the manifold, the air mixes in a central chamber called a plenum. From there, the air is directed to the cylinders through individual intake runners. Some models feature an Exhaust Gas Recirculation (EGR) valve. Under certain conditions, this valve introduces a small amount of exhaust gas into the air/fuel mixture to cool the combustion process and lower emissions. Six-cylinder Subaru models include a Variable Intake Control Valve or Iris Valve to increase the versatility of the intake manifold for improved drivability at all speeds.
The diameter and length of each runner in the manifold are initially determined by computer. Every effort is made to ensure each runner is exactly the same length, with smooth turns. This allows for a smooth, even flow of air to the cylinders.
In some cases, however, a smooth flow of air isnt the best thing. Selected Subaru models feature an intake that tumbles the incoming air on purpose. This is the case with the high-performance Impreza WRX. Its intake includes tumble generator valves directly above the fuel injectors. When the engine is at idle, these valves close and bypass the incoming airflow through a passage to increase the swirl of the air/fuel mixture, resulting in an engine that operates more cleanly. Other benefits include easier cold starts, reduced emissions and efficient low-speed operation. In fact, despite its startling power and acceleration, the WRX engine burns so cleanly it qualifies as a Low Emission Vehicle (LEV) under todays EPA guidelines.
Such carefully designed systems are ultimately developed by thoughtful Subaru engineers like Kazuei Itoh.
Itoh, who has been involved in the research and development of engine controls for the past 11 years, designed the intake manifold for the naturally aspirated (non-turbocharged) six-cylinder horizontally opposed engine mounted in the Outback H6-3.0. Working on the manifold gave Itoh a deeper appreciation of the intake manifolds flexibility and overall importance as part of the Subaru engine and driving experience.
That project was a rare and valuable opportunity for me, Itoh says. The aim was not only to increase the engines power, torque and fuel efficiency, but also to provide an enjoyable ride for the driver and passengers. In this role, the intake manifold is very important.